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The coordination number of atoms in an FCC metal structure is 12. This is because:

1. Each atom touches 4 others in the same plane, 3 in the plane above, and 3 in the plane below
2. Each atom touches 4 others in the same plane, 4 in the plane above, and 4 in the plane below
3. Each atom touches 6 others in the same layer, 3 in the plane above, and 3 in the plane below
4. Each atom touches 3 others in the same layer, 6 in the layer above, and 6 in the layer below

Correct answer: Each atom touches 4 others in the same plane, 4 in the plane above, and 4 in the plane below

Solution

In FCC close-packing each atom is in contact with 12 neighbours: 4 in the same close-packed layer, 4 in the layer above, and 4 in the layer below, giving a total coordination number of 12.

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